Osama bin Laden's al-Qaeda terrorist network has been active in the Balkans for years, most recently helping Kosovo rebels battle for independence from Serbia with the financial and military backing of the United States and NATO.

The claim that al-Qaeda played a role in the Balkan wars of the 1990s came from an alleged FBI document former Yugoslav leader Slobodan Milosevic presented in his defence before the Hague tribunal last week. Mr. Milosevic faces 66 counts of war crimes and genocide.

Although Hague prosecutors have challenged the veracity of the document, which Mr. Milosevic identified as a Congressional statement from the FBI dated last December, Balkan experts say the presence of al-Qaeda militants in Kosovo and Bosnia is well documented.

Today, al-Qaeda members are helping the National Liberation Army, a rebel group in Macedonia, fight the Skopje government in a bid for independence, military analysts say. Last week, Michael Steiner, the United Nations administrator in Kosovo, warned of "importing the Afghan danger to Europe" because several cells trained and financed by al-Qaeda remain in the region.

"Many members of the Kosovo Liberation Army were sent for training in terrorist camps in Afghanistan," said James Bissett, former Canadian ambassador to Yugoslavia and an expert on the Balkans. "Milosevic is right. There is no question of their participation in conflicts in the Balkans. It is very well documented."

The arrival in the Balkans of the so-called Afghan Arabs, who are from various Middle Eastern states and linked to al-Qaeda, began in 1992 soon after the war in Bosnia. According to Lenard Cohen, professor of political science at Simon Fraser University, mujahedeen fighters who travelled to Afghanistan to resist the Soviet occupation in the 1980s later "migrated to Bosnia hoping to assist their Islamic brethren in a struggle against Serbian [and for a time] Croatian forces."

The Bosnian Muslims welcomed their assistance. After the Bosnian war, "hundreds of Bosnian passports were provided to the mujahedeen by the Muslim-controlled government in Sarajevo," said Prof. Cohen in a recent article titled Bin Laden and the war in the Balkans. Many al-Qaeda members decided to stay in the region after marrying local Muslim women, he said.

They also set up secret terrorist training camps in Bosnia -- activities financed by the sale of opium produced in Afghanistan and secretly shipped through Turkey and Kosovo into central Europe.

In the years immediately before the NATO bombing of Yugoslavia in 1999, the al-Qaeda militants moved into Kosovo, the southern province of Serbia, to help ethnic Albanian extremists of the KLA mount their terrorist campaign against Serb targets in the region.

The mujahedeen "were financed by Saudi and United Arab Emirates money," said one Western military official, asking anonymity. "They were mercenaries who were not running the show in Kosovo, but were used by the KLA to do their dirty work."

The United States, which had originally trained the Afghan Arabs during the war in Afghanistan, supported them in Bosnia and then in Kosovo. When NATO forces launched their military campaign against Yugoslavia three years ago to unseat Mr. Milosevic, they entered the Kosovo conflict on the side of the KLA, which had already received "substantial" military and financial support from bin Laden's network, analysts say.

In the wake of the Sept. 11 terrorist strikes on the United States, NATO began to worry about the presence in the Balkans of the Islamist terrorist cells it had supported throughout the 1990s.